An urchin barren is an area of the subtidal where the population growth of sea urchin

Sea urchin

Sea urchins or urchins are small, spiny, globular animals which, with their close kin, such as sand dollars, constitute the class Echinoidea of the echinoderm phylum. They inhabit all oceans. Their shell, or "test", is round and spiny, typically from across. Common colors include black and dull...

s has gone unchecked, causing destructive grazing of kelp beds or kelp forest

Kelp forest

Kelp forests are underwater areas with a high density of kelp. They are recognized as one of the most productive and dynamic ecosystems on Earth. Smaller areas of anchored kelp are called kelp beds....

s (specifically the giant brown bladder kelp, Macrocystis

Macrocystis

Macrocystis is a genus of kelp . This genus contains the largest of all the phaeophyceae or brown algae. Macrocystis has pneumatocysts at the base of its blades. Sporophytes are perennial, and individual stipes may persist for many years...

).

Sea urchins eat kelp holdfasts but also climb on kelp to graze on the stipes and fronds. This can be caused by a lack of sea otter

Sea Otter

The sea otter is a marine mammal native to the coasts of the northern and eastern North Pacific Ocean. Adult sea otters typically weigh between 14 and 45 kg , making them the heaviest members of the weasel family, but among the smallest marine mammals...

s or other predators in the area, which makes it extremely important to protect the ecological balance in a kelp forest.